Stabilization of proteins in solution

ABSTRACT

A protein stabilizer additive comprises two or more of a tris compound of the formula (1): (HOCH 2 ) 3  --C--R, wherein R is: C 1  -C 4  alkyl, substituted C 1  -C 4  alkyl, NH 2  ; NR 1  R 2  wherein R 1  and R 2  may be independently: H, C 1  -C 4  alkyl sulphonate, C 1  -C 4  hydroxyalkyl sulphonate; C 1  -C 4  alkyl NHC(CH 2  OH) 3 , C 1  -C 4  alkyl, C 1  -C 4  hydroxyalkyl; C 1  -C 4  alkyl carboxylate; a polyelectrolyte; a buffer; and one or more additional components for example divalent metal salts.

This application is a 371 of PCT/GB94/02180 filed Oct. 6, 1994.

BACKGROUND OF THE INVENTION

This invention relates to stabilization of proteins in solution, particularly but not exclusively to stabilisation of enzymes. Alternative proteins include antibodies, antigens, serum compliment, vaccine components and bioactive peptides.

Use of enzymes in analytical applications has become well known because enzymes afford a number of significant advantages over conventional analytical chemistry. Enzymes confer specificity, sensitivity and operate under mild analytical conditions. A major disadvantage of enzyme based assays is that the enzyme component is often unstable. This may lead to degeneration of the reagent during storage and spurious results. Various methods have been used to increase the stability of enzymes including immobilisation, chemical modification by cross-linking, polymer grafting or substitution reactions, physical entrapment or encapsulation in polymer matrices or membranes and the addition of chemicals or solvents to the enzyme preparation. Enzyme preparations for use in analytical methods are often supplied in a dry stabilized form using a combination of chemical additives to promote stability. WO90/05182 and WO91/14773 disclose stabilization of enzymes on drying by mixing aqueous solutions of the enzyme with soluble polyelectrolytes and cyclic polyols before removal of water from the solution. Such compositions have not been found to afford significant stabilization prior to dehydration.

According to a first aspect of the present invention a protein stabilizer additive comprises two or more of:

SUMMARY OF THE INVENTION

a. a tris(hydroxymethyl)methyl compound of formula 1;

    (HOCH.sub.2).sub.3 C--R                                    (1)

wherein R is: C₁ -C₄ alkyl, substituted C₁ -C₄ alkyl, NH₂ ; NR¹ R² wherein R¹ and R² may be independently: H, C₁ -C₄ alkyl sulfonate, C₁ -C₄ hydroxyalkyl sulfonate; C₁ -C₄ alkyl-NHC(CH₂ OH)₃, C₁ -C₄ alkyl, C₁ -C₄ hydroxyalkyl; C₁ -C₄ alkyl carboxylate;

b. a polyelectrolyte;

c. a buffer; and

d. one or more additional components.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Component (a) may be referred to as a "tris" compound. Examples of "tris" compounds include: 1,1',1"-tris(hydroxymethyl)ethane; 1,1',1"-tris(hydroxymethyl)propane; tris(hydroxymethyl)aminomethane or salts thereof for example chloride, maleate, phosphate, succinate salts; 1,3-bis[tris(hydroxymethyl)methylamino]propane; bis(2-hydroxyethyl)amino-tris(hydroxymethyl)methane; N-[tris(hydroxymethyl)methyl]-2-aminoethane sulphonate; N-[tris(hydroxymethyl)methyl]-3-aminopropane sulphonate; N-[tris(hydroxymethyl)methyl]-3-amino-2-hydroxypropane sulphonate; N-[tris(hydroxymethyl)methyl]-glycine.

The polyelectrolyte may be a cationic or anionic polyelectrolyte. Amphoteric polyelectrolytes may also be employed. The cationic polyelectrolyte is preferably a polymer with cationic groups distributed along the molecular chain. The cationic groups, which are preferably quaternary ammonium derived functions, may be disposed in side groups pendant from the chain or may be incorporated in it. Examples of cationic polyelectrolytes include: Coplymers of vinyl pyrollidone and quaternary methyl methacrylate e.g., GAFQUAT® series (755N, 734, HS-100) obtained from ISP; substituted polyacrylamides; polyethyleneimine, polypropyleneimine and substituted derivatives; polyamine homopolymers (GOLCHEM® CL118); polyamine co-polymers (e.g., condensates of epichlorohydrin and mono or dimethylamine); polydiallyl dimethyl ammonium chloride (polyDADMAC); substituted dextrans; modified guar gum (substituted with hydroxypropytrimonium chloride); substituted proteins (e.g., quaternary groups substituted on soya protein and hydrolysed collagen); polyamino acids (e.g., polylysine); low molecular weight polyamino compounds (e.g., spermine and spermidine). Natural or artificial polymers may be employed. Cationic polyelectrolytes with MW 150 to 5,000,000, preferably 5000 to 500,000, more preferably 5000 to 100,000 may be employed. An amount of 0.01 to 10% is preferred, more preferably 0.1 to 2% w/v, especially 0.05 to 5%.

The anionic polyelectrolyte is preferably a polymer with anionic groups distributed along the molecular chain. The anionic groups, which may include carboxylate, sulfonate, sulphate or other negatively charged ionisable groupings, may be disposed upon groups pendant from the chain or bonded directly to the polymer backbone. Natural or artificial polymers may be employed.

Examples of anionic polyelectrolytes include: Gantrez (S-series, AN-series); alginic acid and salts; carboxymethyl celluloses and salts; substituted polyacrylamides (eg substituted with carboxylic acid groups); polyacrylic acids and salts; polystyrene sulfonic acids and salts; dextran sulphates; substituted saccharides e.g., sucrose octosulfate; heparin. Anionic polyelectrolytes with MW of 150 to 5,000,000 may be used, preferably 5000 to 500,000, more preferably 5000 to 100,000. An amount of 0.01% to 10% is preferred especially 0.05 to 5% more especially 0.1 to 2% w/v.

The said further component may be selected from the group comprising divalent metal ions, chelators for example EDTA, EGTA or citrate (not with peroxidases) or polyols. Preferred divalent metals include calcium and magnesium salts. Cobalt, zinc or manganese salts may also be employed.

The polyols which may be employed are preferably low molecular weight polyols although polymeric derivatives may be employed. Preferred polyols lower the dielectric of the solution. Such polyols include ethylene glycol, glycerol, erythritol and mannitol. Cyclic polyols which may be employed incorporate one or more alicyclic rings and may have at least one side chain. Preferred cyclic polyols include disaccharides and sugar alcohols, for example lactitol, sorbitol and inositol. Compounds having 2 to 10 hydroxyl groups are preferred. The amount of the polyol may be in the preferred range 1 to 5% more preferably 1 to 20% most preferably 2 to 10% w/v.

Compositions of the present invention stabilize enzymes or other proteins without covalent or otherwise irreversible binding to the latter. The enzymes may be recovered intact from the solution by simple physical means, for example by adjustment of pH to a suitable value followed by salt or solvent precipitation, conveniently with ammonium sulphate.

Compositions of the present invention preferably consist essentially of one or more enzymes or other proteins together with buffers and stabilizers as described in the specification. Naturally occurring complex mixtures such as plasma, serum or other physiological fluids, which may include polyelectrolytes, hydroxy compounds and salts are excluded from the present invention. However immobilised, cross-linked, entrapped or covalently linked proteins are included within the present invention.

Compositions of the present invention are considered to stabilize a protein if the activity of the protein is not significantly diminished after a period of incubation at elevated temperatures in comparison to the protein in the absence of the stabilizers. For example horseradish peroxidase incubated at 60° C. for 120 minutes shows no activity loss with stabilizers of this invention compared to 50% activity loss at 18 minutes under the same conditions.

According to a second aspect of the present invention a method of stabilizing a protein includes the step of contacting the protein with an aqueous solution of an additive as described above.

According to a third aspect of the present invention there is provided use of an additive in accordance with the first aspect of this invention for stabilizing an aqueous protein solution.

The present invention finds particular application in the preparation of analytical assay apparatus. A preferred aspect of the present invention provides an analytical assay formulation incorporating a stabilizer additive as defined above.

The invention is further described by means of example but not in any limitative sense.

The stability of protein solutions in the presence of stabilisers at elevated temperatures was investigated. Buffer solutions containing stabilizers were incubated to the required temperature in a Techne dry heating block. After several minutes incubation the temperature of the buffered mixture was measured using a thermistor. When the temperature was constant at the required level, protein solution was added and the tube was quickly inverted to mix thoroughly and returned to the dry block. Samples were taken at fixed time points thereafter and assayed for activity by standard procedures. All of the results were expressed as the amount of protein activity relative to the zero time activity. Zero time samples were obtained by incubation of the system at 25° C., duplicate samples being taken and assayed for protein activity

The present invention is demonstrated by reference to the data in Tables 1 to 20 which show relative specific activities of various proteins as a function of time. Solutions of the proteins without stabilizers underwent rapid denaturation as shown by loss of activity whereas with a polyelectrolyte present greater activity was retained for longer periods. Inclusion of one or more "tris" compounds gave a further increase in stability with the protein activity being retained. Tables 1 and 2 show the effects of a polyelectrolyte (Gafquat 755N), tris compounds and EDTA (as a metal chelator) on the solution stability of alcohol oxidase at 60° C.

                  TABLE 1                                                          ______________________________________                                         Effect of Stabilizers on Alcohol Oxidase Activity in Solution                         % Remaining Enzyme Activity                                                             Polyelec-        +Tris  +Tris                                    Time  trolyte +Tris ethane propane                                             (Min) Control Alone 89 mM 89 mM 89 mM                                        ______________________________________                                          0     100.00   100.0     100.0  100.0  100.0                                     5 54.2 113.0 116.0 138.0 138.0                                                10 2.1 100.0 111.0 107.0 138.0                                                 15 0 80.2 92.1 95.0 123.0                                                      20 0 64.5 81.6 74.9 104.0                                                      25 0 58.0 78.9 76.4 115.0                                                      30 0 38.9 26.3 59.1 96.2                                                     ______________________________________                                          The buffer used in this example was 200 mM phosphate pH 8.0.             

                  TABLE 2                                                          ______________________________________                                         Effect of Tris and EDTA with Alcohol Oxidase                                           % Remaining Enzyme Activity                                                                       +Tris                                                   Polyelec- 89 mM and                                                          Time  trolyte EDTA +Tris                                                       (Min) Control Alone 1.99 mM no EDTA                                          ______________________________________                                          0      100.0   100.0      100.0   100.0                                          5 61.0 113.0 162.0 116.0                                                      10 15.3 100.0 168.0 111.0                                                      15 10.2 80.2 154.0 92.1                                                        20 6.8 64.5 134.0 81.6                                                         25 6.8 58.0 140.0 78.9                                                         30 5.1 38.9 123.0 26.3                                                       ______________________________________                                    

The enzyme was alcohol oxidase from Hansenula polymorpha (50 units ml⁻¹). The tris solution was buffered to pH 8.0 with phosphoric acid.

The polyelectrolyte was GAFQUAT® 755N (1% w/v).

The enzyme solution was thermally stressed at 60° C. for 30 minutes with the recorded values being the percentage of remaining enzyme activity at 5 minute intervals during the incubation.

                  TABLE 3                                                          ______________________________________                                         Horseradish Peroxidase in 20 mM Tris pH 8.0                                          % Remaining Enzyme Activity                                                    No       Gafquat   Ethylene                                                                               Gafquat 755 N                                   Time Stabil- 755 N Glycol 0.5 w/v + Ethylene                                   (Min) izers 0.5% w/v 10% v/v Glycol 10% v/v                                  ______________________________________                                          0    100      100       100     100                                              5 77.6 88.1 72.6 90.8                                                         10 69.1 76.2 63.4 78.1                                                         15 57.8 63.5 57.1 70.75                                                        20 48.1 59.3 52.5 63.4                                                         25 46.0 53.6 48.5 59.25                                                        30 39.3 48.9 43.8 55.1                                                         40  46.5 41 50.7                                                             ______________________________________                                    

Table 3 shows stabilization of horseradish peroxidase in 20 mM tris at pH 8.0, at a temperature of 60° C. The combination of the cationic polymer gafquat 755N and ethylene glycol produced better stabilization than either components alone.

                  TABLE 4                                                          ______________________________________                                         Horseradish Peroxidase in 20 mM Tris Buffer pH 8.0                                     % Remaining Enzyme Activity                                                                            Gafquat 755 N                                     No Gafquat  0.5% + Ethylene                                                   Time Stabil- 755 N CaCl.sub.2 Glycol 10% v/v +                                 (Min) izers 0.5% 10 mM CaCl.sub.2 10 mM                                      ______________________________________                                          0      100     100       100   100                                               5 77.6 88.1 81.4 99.3                                                         10 69.1 76.2 85.2 96                                                           20 48.1 59.3 83.4 99.3                                                         30 39.3 48.9 80.8 96                                                           60   84.2 95                                                                   90   74.8 84.5                                                                 120    80.1 87.1                                                               180    72.6 92.1                                                               210    67.2 83.4                                                               240     84.9                                                                 ______________________________________                                    

Table 4 shows stabilization of horseradish peroxidase over an extended period at a temperature of 60° C. Calcium chloride alone produced good stabilisation but a combination of cationic polyelectrolyte, ethylene glycol and calcium chloride provided a high level of stabilization for up to 240 minutes.

                  TABLE 5                                                          ______________________________________                                         Arthromyces Peroxidase                                                                    % Remaining Enzyme Activity                                         20 mM Bis-Tris                                                                                                         GAFQUAT ®                             Phosphate No GAFQUAT ®  755 N                                             Time 20 mM No Stabil- 755 N CaCl.sub.2 0.5% w/v +                              (Min) Stabilizers izers 0.5% w/v 10 mM CaCl.sub.2 10 mM                      ______________________________________                                          0    100      100     100      100   100                                         5 68.3 97.2                                                                   10 63.4 92.6                                                                   20  85.53 96 96.8 98.4                                                         40 46.3 71.4 82 84 88.6                                                        60   72 73.2 80.9                                                              150    36 42 52                                                              ______________________________________                                    

Table 5 shows stabilization of arthromyces peroxidase at 4.5 Uml⁻¹ in 20 mM bis-tris at pH 7.3 at a temperature of 59° C. Stabilization was obtained with the cationic polyelectrolyte gafquat 755N and also with calcium chloride. However superior stabilization was obtained with a combination of both stabilizers.

                  TABLE 6                                                          ______________________________________                                         Pig Liver esterase. Gafquat/EDTA                                                     % Remaining Enzyme Activity                                              Time  No Stabil-                                                                              GAFQUAT ®                                                                            EDTA  GAFQUAT ® 0.5% w/v                            (Min) izers 0.5% w/v 5 mM EDTA 5 mM                                          ______________________________________                                          0    100      100       100   100                                                5 45.8 58.8 52.8 62.4                                                         10 21.2 36 40.3 41.8                                                           15 13.4 24 30 31.6                                                             25 3.7 9.4 10 8.7                                                              35  5.6  4.9                                                                 ______________________________________                                    

Table 6 shows stabilization of pig liver esterase with gafquat and EDTA in 20 mM bis-tris pH 7.3 and an incubation temperature of 68.9° C.

                  TABLE 7                                                          ______________________________________                                         Pig Liver Esterase. Tris (Hydroxymethyl) ethane.                                      % Remaining Enzyme Activity                                                     Time   No Stabil-  Tris (OHMe)                                                                            Tris (OHMe)                                   (Min) isers Et 1% Et 0.1%                                                    ______________________________________                                          0     100           100       100                                                5 45.8 60.5 65.1                                                              10 21.2 26 39.6                                                                15 13.4 15.2 26                                                                25 3.7 4.6 10.2                                                              ______________________________________                                    

Table 7 shows stabilisation of pig liver esterase in 20 mM Bis-Tris pH 7.3 with tris (hydroxymethyl) ethane at 68.9° C.

                  TABLE 8                                                          ______________________________________                                         Stabilisation of Horseradish Peroxidase During Dilution in                       20 mM Tris Buffer pH 8.0 at a Temperature of 25° C.                          % Relative Specific Activity                                                                              DEAE-Dextran                                       MW 500 K (0.5%                                                             HRP-4   DEAE- w/v) + Ethylene                                                  Concen-   Dextran Glycol                                                       tration No CaCl.sub.2 MW 500 K (10% v/V) +                                     (ug/ml) Additive (10 mM) (0.5% w/v) CaCl.sub.2 (10 mM)                       ______________________________________                                         0.732  15       80         98     100                                            1.83 44 90  99  98                                                             3.66 64 95 100 102                                                             7.32 103  97 100 101                                                           18.3 97 105  102 101                                                           36.6 100  98 101  99                                                         ______________________________________                                    

Table 8 shows the stabilisation of horseradish peroxidase in 20 mM tris at pH 8.0. DEAE-dextran both alone and in the presence of ethylene glycol and calcium chloride surprisingly afforded stability at extreme dilutions.

                  TABLE 9                                                          ______________________________________                                         The Stability of Dilute Solutions of Horseradish                                 Peroxidase (3.66 ug/ml) At 37° C.                                            % Remaining Enzyme Activity                                                              DEAE-      DEAE-Dextran MW 500 K                                 No Dextran (0.5% w/V) + Ethylene                                              Time Stabil- MW 500 K Glycol (10% v/v) +                                       (Min) isers (0.5% w/v) CaCl.sub.2 (10 mM)                                    ______________________________________                                          0     100       100        100                                                   5 62 80  98                                                                   10 56 79 101                                                                   15 51 77 102                                                                   20 47 81  98                                                                   30 40 79 103                                                                   40 35 75 101                                                                   50 30 76 103                                                                   60 23 75 101                                                                   90  71 100                                                                     120   70  96                                                                   150   65 106                                                                   180   60 103                                                                 ______________________________________                                    

Table 9 shows stability of very dilute solutions of horseradish peroxidase (3.66 μg/ml⁻¹) at 37° C. A combination of DEAE-dextran, ethylene glycol and calcium chloride afforded excellent stabilisation for up to 180 minutes.

                  TABLE 10                                                         ______________________________________                                         Trypsin Autodigestion (Phosphate Buffer)                                              % Remaining Enzyme Activity                                                     Time   No        Gafquat 755 N                                                                           DEAE-Dextran                                   (Min) Stabilisers 0.5% w/v 0.5% w/v                                          ______________________________________                                          0     100         100        100                                                 5 55.6 82.8 66.7                                                              10 40.2 61.8 57.2                                                              15 35.5 50.6 45.7                                                              20 26.4 48.3 42.3                                                              35                                                                           ______________________________________                                    

Tables 10 and 11 show that the presence of polyelectrolyte retards autodegradation of trypsin, this effect was enhanced in the presence of tris stabiliser.

                  TABLE 11                                                         ______________________________________                                         Trypsin Autodigestion (Tris Buffer)                                                   % Remaining Enzyme Activity                                                     Time   No       GAFQUAT ® 755 N                                                                        DEAE-Dextran                                 (Min) Stabilisers 0.5% w/v 0.5% w/v                                          ______________________________________                                          0     100        100           100                                               5 72.3 95 90.8                                                                10 54.8 82.8 73.6                                                              15 50.5 69 60.9                                                                20 25.4 57.1 54.3                                                              35 17.7 40 39.6                                                              ______________________________________                                    

                  TABLE 12                                                         ______________________________________                                         Alkaline Phosphatase (Bovine)                                                             % Remaining Enzyme Activity                                                             MgCl.sub.2 10 mM Ethylene                                      Glycol (10% v/v,                                                             Time No 0.5% w/v) BSA and                                                      (Min) Stabilisers DEAE-Dextran (0.5 w/v)                                     ______________________________________                                          0         100      100                                                           5 61.4 88.5                                                                   10 45.7 72.5                                                                   15 42.1 66.4                                                                   20 36.8 61.9                                                                   40 30.1 50                                                                   ______________________________________                                    

Table 12 shows stabilisation of bovine alkaline phosphatase in 50 mM tris buffer at pH 8.0 and 61° C. Stabilisers comprising magnesium chloride, ethylene glycol, Bovine Serum Albumin and DEAE-dextran provided enhanced stabilisation.

                                      TABLE 13                                     __________________________________________________________________________     The Effect of Combinations of Stabilisers on the Stability of Horseradish       Peroxidase(Biozyme) Solutions (20 mM Tris/HCl Buffer pH 8.0) at 69.degree     . C.                                                                               % Remaining Enzyme Activity                                                                                            DEAE-Dextran                               DEAE- MW 500K                                                                 Ethylene Dextran MW (0.5% w/v) +                                               Glycol 500K (0.5 Ethylene                                                   DEAE-Dextran Ethylene  (10% v/v) + w/v) + Glycol (10%                        Time No MW 500K Glycol CaCl.sub.2 CaCl.sub.2 CaCl.sub.2 v/v) + CaCl.sub.                                                 2                                    (Min) Stabiliser (0.5% w/v) (10% v/v) (10 mM) (10 mM) (10 mM) (10            __________________________________________________________________________                                                 mM)                                 0  100   100     100   100   100    100    100                                   5 78 88 94                                                                    10 69 75 76                                                                    15 58 67 64                                                                    20 48 61 61 98 103  99 101                                                     25 45 55 57                                                                    30 39 50 53                                                                    40    78 101  99 103                                                           60    97 101 100 101                                                           80    92 100  97 102                                                           120     85  96  94 103                                                         180     79  90  85  97                                                         240        80  92                                                            __________________________________________________________________________

Table 13 shows the effect of combinations of stabilisers on the stability of horseradish peroxidase in 20 mM tris/HCl buffer at pH 8.0 and 69° C. This example shows the use of substituted dextrans as polyelectrolytes, a good stabilisation chloride in combination.

                  TABLE 14                                                         ______________________________________                                         The Stability of HRP-4 (Biozyme) Solutions (20 mM                                Tris/HCI Buffer pH 8.0) at 50° C.                                                  % Remaining Enzyme Activity                                                             DEAE-Dextran MW                                              500 K (0.5% w/v) +                                                             Ethylene Glycol                                                              Time No (10% v/v) + CaCl.sub.2                                                 (Days) Stabiliser (10 mM)                                                    ______________________________________                                         0            100                                                                 0.0347 90.3                                                                    0.056 95.1                                                                     0.0764 90.3                                                                    0.0972 82                                                                      0.118 69.4                                                                     0.139 65.4                                                                     0.167 63.8                                                                     0.26 46.3                                                                      0.27 49.3                                                                      0.29 38.4 101                                                                  1                                                                              2  99                                                                          6  106                                                                         8  102.8                                                                     ______________________________________                                    

Table 14 shows stabilisation of horseradish peroxidase solutions in 20 mM tris/HCl buffer at pH 8 and 50° C. Degradation without stabiliser is rapid but good stabilisation was obtained at periods up to 8 days using a combination of DEAE-dextran, ethylene glycol and calcium chloride.

                  TABLE 15                                                         ______________________________________                                         The Effect of Temperature on the Stability of HRP-4                              (Biozyme) Solutions (20 mM Tris/HCI Buffer pH 8.0) in the                      Presence of Ethylene Glycol (10% v/v), DEAE-Dextran                            MW 500 K (0.5% w/v) and Calcium Chloride (10 mM)                                      % Remaining Enzyme Activity                                           Time (Min)                                                                              85.5° C.                                                                          80.5° C.                                                                          75.5° C.                                                                        70.5° C.                           ______________________________________                                         0        100       100       100     100                                         3 71                                                                           5  80 91                                                                       6 56                                                                           9 39                                                                           10  64 85 102                                                                  12 30                                                                          15 28 53 82                                                                    18 25                                                                          20  47 77 100                                                                  21 22                                                                          25  40                                                                         30  33 73 103                                                                  40  30 68                                                                      50  25 63 100                                                                  60   58                                                                        70     99                                                                      110    100                                                                     130     93                                                                     150     93                                                                     170     88                                                                   ______________________________________                                    

Table 15 shows the effect of temperature on the stability of horeradish peroxidase solutions (20 mM tris/HCl buffer pH 8.0) in the presence of ethylene glycol, DEAE-dextran (MW 500K) and calcium chloride.

                  TABLE 16                                                         ______________________________________                                         Long Term Stability of HRP-4 Solutions                                                % Remaining Enzyme Activity                                                                        No                                                     No  Stabiliser; Stabilisation                                                 Time Stabiliser; Stabilisation Room Buffer; Room                               (Days) 37° C. Buffer; 37° C. Temperature Temperature           ______________________________________                                          0     100      100        100      100                                           6 90 101  96  99                                                              15 85 98 93 100                                                                21 76 99 86 101                                                                33 32 99 82 101                                                              ______________________________________                                    

Table 16 illustrates long term stabilisation of horseradish peroxidase solutions at different temperatures. The stabilisation buffer was the same as for example 15.

                  TABLE 17                                                         ______________________________________                                         The Stabilisation of HRP Activity of Antibody/HRP Conjugate                      (Sigma) Solutions (20 mM Tris/HCI Buffer pH 8.0) at 50° C.                   % Remaining Enzyme Activity                                                                            DEAE-Dextran MW                                     DEAE-Dextran 500K (0.5% w/v) +                                                 MW 500K (0.5% CaCl.sub.2 (10 mM) +                                           Time  w/v) + CaCl.sub.2 Ethylene Glycol                                        (Hours) No Stabiliser (10 mM) (10% v/v)                                      ______________________________________                                         0      100        100          100                                               0.5 86 99.4 100.6                                                              20.8 35.9 80.1 100                                                             48   99                                                                        72   97.6                                                                    ______________________________________                                    

Table 17 illustrates stabilisation of horseradish peroxidase activity of antibody-horseradish peroxidase conjugate solutions using the following stabiliser: CaCl₂ 10 mM, ethylene glycol 10% v/v, DEAE-dextran 0.5% w/v, Buffer Tris/HCl 20 mM pH 8.0

The stabilisation of the HRP label of the IgG-HRP conjugate (SIGMA® A 6029) with the combination described resulted in loss of activity over 3 days incubation of 50°C.

                  TABLE 18                                                         ______________________________________                                         Galactose Oxidase: Stability at 66.5° C. in 20 mM Tris                    phosphate pH 7.84                                                                         % Remaining Enzyme Activity                                                              Gantrez S-97                                              Time No Stabiliser 0.5% w/v                                                  ______________________________________                                          0           100       100                                                        5 87.9 96.1                                                                   10 69.5 82.1                                                                   15 58.6 75.6                                                                   20 53.2 68                                                                   ______________________________________                                    

                  TABLE 19                                                         ______________________________________                                         Galactose Oxidase: Stability at 66.4° C. in 20 mM Tris                    phosphate pH 7.84                                                                        % Remaining Enzyme Activity                                                               Sodium Alginate                                           Time No Stabilisers 0.2% w/v                                                 ______________________________________                                          0          100        100                                                        5 87.9 114.5                                                                  10 69.5 102.8                                                                  15 58.6 99.8                                                                   20 53.2 79.5                                                                 ______________________________________                                    

                  TABLE 20                                                         ______________________________________                                         Alcohol Oxidase in 20 mM Bis-Tris pH 6.0                                                   % Remaining Enzyme Activity                                                               Carboxymethyl                                             Time No Stabilisers Cellulose 0.125%                                         ______________________________________                                          0          100        100                                                        5 15 37.1                                                                     10 10.7 27.9                                                                   20 8.9 21.8                                                                    30 7 19                                                                      ______________________________________                                     

What is claimed is:
 1. A method of stabilizing an aqueous enzyme comprising the step of contacting the enzyme selected from the group consisting of peroxidase enzymes, alkaline phosphatase enzymes and antibody conjugates thereof with an aqueous solution of a protein stabilizer additive comprising:a. a tris(hydroxymethyl)methyl compound of formula 1;

    (HOCH.sub.2).sub.3 C--R                                    (1)

wherein R is: NH₂ ; NR¹ R² wherein R¹ and R² may be independently: H, C₁ -C₄ alkyl, C₁ -C₄ hydroxyalkyl, C₁ -C₄ alkyl sulfonate, C₁ -C₄ hydroxyalkyl sulphonate; C₁ -C₄ alkyl NHC(CH₂ OH)₃ ; C₁ -C₄ alkyl carboxylate; b. a cationic polyelectrolyte selected from the group consisting of copolymers of vinyl pyrollidone and quaternary methyl methacrylate and DEAE dextran; c. a polyol selected from the group consisting of glycerol, ethylene glycol, sorbitol and lactitol; and d. one or more salts selected from the group consisting of salt of calcium, magnesium and cobalt.
 2. A method as claimed in claim 1 wherein the tris(hydroxymethyl)) methyl compound is selected from the group consisting of: tris(hydroxymethyl)aminomethane and salts thereof selected from the group consisting of chloride, maleate, phosphate, succinate salts; 1,3-bis{tris(hydroxymethyl)methylamino}propane; bis(2-hydroxyethyl)amino-tris(hydroxymethyl)methane; N-{tris(hydroxymethyl)methyl}-2-aminoethane sulfonate; N-{tris(hydroxymethyl)methyl}-3-aminopropane sulfonate; N-{tris(hydroxymethyl)methyl}-3-amino-2-hydroxypropane sulfonate; and N-{tris(hydroxymethyl)methyl}-glycine.
 3. A method as claimed in claim 1 wherein the cationic polyelectrolyte has a molecular weight of 150 to 5,000,000 Daltons.
 4. A method as claimed in claim 1 wherein the cationic polyelectrolyte is present in an amount of 0.01 to 10% w/v.
 5. A method as claimed in claim 1 wherein the polyol is present in an amount of 0.1 to 25% w/v.
 6. A method as claimed in claim 1 wherein said peroxidase enzyme is selected from the group consisting of horseradish peroxidase and arthromyces peroxidase.
 7. A method as claimed in claim 2, wherein the tris(hydroxymethyl) compound is selected from the group consisting of tris(hydroxymethyl)aminomethane and salts thereof selected from the group consisting of chloride, maleates, phosphate, succinate salts; 1,3-bis{tris(hydroxymethyl)methylamino}propane; and bis(2-hydroxyethyl)aminotris(hydroxymethyl)) methane.
 8. A method as claimed in claim 1, wherein said enzyme is horseradish peroxidase, said tris is 20 mM tris/HCL buffer pH 8.0, said polyol is ethylene glycol, said cationic polyelectrolyte is DEAE-dextran (MW 500K), and said salt is calcium chloride.
 9. A method as claimed in claim 8, wherein at least 99% of the enzyme's activity is retained after 33 days at 37° C. 